Mice bearing primary MCA-induced sarcomas are capable of generating normal or near normal cytotoxic cellular immune responses, using as a test primary in vitro spleen cell sensitization. Tumor-related immunosuppression does occur when irradiated tumor cells are cocultured with spleen cells during in vitro sensitization. This immunosuppression can be reversed by chemical modification of tumor cells, which also increases in vitro tumor immunogenicity. Primary MCA-induced murine tumor cells have been adapted to rapid flow microfluorometry analysis. This technique has revealed that primary MCA-induced tumor cells stain with fluoresceinated anti-MuLV antisera at intensities undetected by routine fluorescent microscopy. Transfer of tumor cells to tissue culture growth conditions results in marked increases in anti-MuLV staining.